1. Field of the Invention
This invention relates to a sample/hold circuit for a color separation circuit in a solid-state color image pick-up device or the like using solid image sensors such as CCDs (charge coupled device).
2. Description of the Prior Art
In the usual charge transfer device color imager, the point sequential image signal that has been obtained through color coding in color filters, is passed through sample/hold circuits to separate the individual R, G and B color signal components prior to various signal treatments.
In a charge transfer device color imager, in which point sequential color decoding is done with color filters, the individual color signal components in the point sequential image signal are subjected to white balance adjustment, gamma correction and various other signal treatments. A signal processing circuit as shown in FIG. 1 has been extensively used for these signal treatments.
In the signal processing circuit of FIG. 1, which is used in a single chip CCD charge transfer device color imager, the image pick-up section includes a CCD1 as an image sensor, which is point sequential color coded by color filters. The CCD1 is driven by a drive clock signal which is provided from a reference clock pulse generator 11. The CCD1 is color coded by color filters such that odd horizontal scanning lines consist of alternate red and green picture elements and that even horizontal scanning lines consist of alternate green and blue picture elements.
The point sequential image signal that is continuously provided from the CCD1 contains three primary color signal components at a repetition frequency f.sub.0 which is determined by the number of picture elements in the CCD1 (f.sub.0 =3.58 MHz in this example). It is read out from the CCD1 under the control of a driving clock signal at a clock frequency of 2.multidot.f.sub.0, and is supplied through a buffer amplifier 2 to a first sample/hold circuit 3. The first sample/hold circuit 3 effects the sampling and holding of the three primary color signal components in the point sequential image signal under the control of a sampling clock signal at a sampling frequency of 2.multidot.f.sub.0, provided from the reference clock pulse generator 11. A shaped point sequential image signal which has been obtained in the above manner, is supplied to second and third sample/hold circuits 4A and 4B. The second and third sample/hold circuits 4A and 4B separate the three primary color signal components in the shaped point sequential image signal. More particularly, the second sample/hold circuit 4A samples and holds the red and blue signal components in the shaped point sequential image signal under the control of a clock signal at a sampling frequency of f.sub.0 which is supplied from the reference clock pulse generator 11. On the other hand, the third sample/hold circuit 4B samples and holds the green signal component in the shaped sequential image signal under the control of a clock signal at f.sub.0 which is supplied through a phase shifter which shifts the phase by 180.degree..
The green signal obtained from the third sample/hold circuit 4B is coupled to a lowpass filter 5B, in which a sampling clock signal leakage component contained in the signal is removed. The output signal from the lowpass filter 5B is amplified by a constant gain amplifier 6C and then clamped to a predetermined signal level by a clamping circuit 7C before it is subjected to a gamma correction treatment in a gamma correction circuit 8.
The output signal from the second sample/hold circuit 4A which contains the red and blue signals, is coupled to a lowpass filter 5A, in which a sampling clock signal leakage component contained in the signal is removed. The output signal from the lowpass filter 5A is coupled to variable gain amplifiers 6A and 6B for white balance adjustment. The outputs of the amplifiers 6A and 6B are clamped to a predetermined signal level by clamping circuits 7A and 7B. The outputs of the clamping circuits 7A and 7B are fed to a gamma correction circuit 8A for gamma correction. The output of the gamma correction circuit 8A is coupled to a simultaneous signal circuit 9 for conversion into simultaneous signals.
The three primary color signals which have been obtained through the white balance adjustment, clamping treatment and gamma correction treatment in the above manner, are supplied to a matrix circuit 10. The matrix circuit 10 forms the luminance signal and chrominance signals from the input three primary color signals.
The three primary color signals for forming a color television signal usually require a frequency band of approximately 3.5 MHz each. The second and third sample/hold circuits 4A and 4B in the signal processing circuit as described above, are required to effect sampling and holding the point sequential image signal at a sampling clock frequency f.sub.0 which is close to the individual color signal frequencies. The lowpass filters 5A and 5B, which serve to remove the sampling clock signal leakage component contained in the individual color signals separated through the second and third sample/hold circuits 4A and 4B, can pass signals in a frequency range in which the color signal component frequencies exist. Also, they have to have a filter characteristic capable of sufficiently attenuating signals in a frequency range in which the leakage component exist. However, since the color signal component frequencies and the leakage component frequency are close to each other, it is impossible to provide an ideal filter which can attenuate only the leakage component.
In the signal processing circuit of the above construction, adverse effects of the sampling signal leakage component will appear in the subsequent signal processing system, for instance on the clamping operation in the clamping circuits 7A, 7B and 7C. Therefore, it is necessary to provide means which can cope with the aforementioned leakage component.
Usually, a sample/hold pulse leakage component is superimposed upon the output signal from a sample/hold circuit, as in the waveform shown in FIG. 2. This leakage component must be removed through a lowpass filter.
The lowpass filter must be capable of sufficiently attenuating frequencies in the range which covers the leakage component. Where the necessary signal component and leakage component frequencies are close to each other, and ideal filter which is capable of attenuating only the leakage component cannot be obtained so that it is impossible to reliably remove the leakage component.